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Samuel K. Degelia, Xuguang Wang, David J. Stensrud, and David D. Turner

Abstract

Nocturnal convection is often initiated by mechanisms that cannot be easily observed within the large gaps between rawinsondes or by conventional surface networks. To improve forecasts of such events, we evaluate the systematic impact of assimilating a collocated network of high-frequency, ground-based thermodynamic and kinematic profilers collected as part of the 2015 Plains Elevated Convection At Night (PECAN) experiment. For 13 nocturnal convection initiation (CI) events, we find small but consistent improvements when assimilating thermodynamic observations collected by Atmospheric Emitted Radiance Interferometers (AERIs). Through midlevel cooling and moistening, assimilating the AERIs increases the fractions skill score (FSS) for both nocturnal CI and precipitation forecasts. The AERIs also improve various contingency metrics for CI forecasts. Assimilating composite kinematic datasets collected by Doppler lidars and radar wind profilers (RWPs) results in slight degradations to the forecast quality, including decreases in the FSS and traditional contingency metrics. The impacts from assimilating thermodynamic and kinematic profilers often counteract each other, such that we find little impact on the detection of CI when both are assimilated. However, assimilating both datasets improves various properties of the CI events that are successfully detected (timing, distance, shape, etc.). We also find large variability in the impact of assimilating these remote sensing profilers, likely due to the number of observing sites and the strength of the synoptic forcing for each case. We hypothesize that the lack of flow-dependent methods to diagnose observation errors likely contributes to degradations in forecast skill for many cases, especially when assimilating kinematic profilers.

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Michael C. Coniglio, Glen S. Romine, David D. Turner, and Ryan D. Torn

Abstract

The ability of Atmospheric Emitted Radiance Interferometer (AERI) and Doppler lidar (DL) wind profile observations to impact short-term forecasts of convection is explored by assimilating retrievals into a partially cycled convection-allowing ensemble analysis and forecast system. AERI and DL retrievals were obtained over 12 days using a mobile platform that was deployed in the preconvective and near-storm environments of thunderstorms during the afternoon in the U.S. Great Plains. The observation locations were guided by real-time ensemble sensitivity analysis (ESA) fields. AERI retrievals of temperature and dewpoint and DL retrievals of the horizontal wind components were assimilated into a control experiment that only assimilated conventional observations. Using the fractions skill score within 25-km neighborhoods, it is found that the assimilation of the AERI and DL retrievals results in far more times when the forecasts are improved than degraded in the 6-h forecast period. However, statistical confidence in the improvements often is not high and little to no relationships between the ESA fields and the actual changes in spread and skill is found. But, the focus on convective initiation and early convective evolution—a challenging forecast problem—and the fact that frequent improvements were seen despite observations from only one system over a limited period, provides encouragement to continue exploring the benefits of ground-based profilers to supplement the current upper-air observing system for severe weather forecasting applications.

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David M. Loveless, Timothy J. Wagner, David D. Turner, Steven A. Ackerman, and Wayne F. Feltz

Abstract

Atmospheric bores have been shown to have a role in the initiation and maintenance of elevated convection. Previous observational studies of bores have been case studies of more notable events. However, this creates a selection bias toward extraordinary cases, while discussions of the differences between bores that favor convective initiation and maintenance and bores that do not are lacking from the literature. This study attempts to fill that gap by analyzing a high-temporal-resolution thermodynamic profile composite of eight bores observed by multiple platforms during the Plains Elevated Convection at Night (PECAN) campaign in order to assess the impact of bores on the environment. The time–height cross section of the potential temperature composite displays quasi-permanent parcel displacements up to 900 m with the bore passage. Low-level lifting is shown to weaken the capping inversion and reduce convective inhibition (CIN) and the level of free convection (LFC). Additionally, low-level water vapor increases by about 1 g kg−1 in the composite mean. By assessing variability across the eight cases, it is shown that increases in low-level water vapor result in increases to convective available potential energy (CAPE), while drying results in decreased CAPE. Most cases resulted in decreased CIN and LFC height with the bore passage, but only some cases resulted in increased CAPE. This suggests that bores will increase the potential for convective initiation, but future research should be directed toward better understanding cases that result in increased CAPE as those are the types of bores that will increase severity of convection.

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Benjamin A. Toms, Jessica M. Tomaszewski, David D. Turner, and Steven E. Koch

Abstract

On 10 August 2014, a gravity wave complex generated by convective outflow propagated across much of Oklahoma. The four-dimensional evolution of the wave complex was analyzed using a synthesis of near-surface and vertical observations from the Oklahoma Mesonet and Atmospheric Radiation Measurement (ARM) Southern Great Plains networks. Two Atmospheric Emitted Radiance Interferometers (AERI)—one located at the ARM SGP central facility in Lamont, Oklahoma, and the other in Norman, Oklahoma—were used in concert with a Doppler wind lidar (DWL) in Norman to determine the vertical characteristics of the wave complex. Hydraulic theory was applied to the AERI-derived observations to corroborate the observationally derived wave characteristics.

It was determined that a bore-soliton wave packet initially formed when a density current interacted with a nocturnal surface-based inversion and eventually propagated independently as the density current became diffuse. The soliton propagated within an elevated inversion, which was likely induced by ascending air at the leading edge of the bore head. Bore and density current characteristics derived from the observations agreed with hydraulic theory estimates to within a relative difference of 15%. While the AERI did not accurately resolve the postbore elevated inversion, an error propagation analysis suggested that uncertainties in the AERI and DWL observations had a negligible influence on the findings of this study.

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Daniel C. Hartung, Jason A. Otkin, Jonathan E. Martin, and David D. Turner

Abstract

The evolution of an undular bore and its associated wind shift, spawned by the passage of a shallow surface cold front over the Southern Great Plains of the United States, is examined using surface and remote sensing observations along with output from a high-resolution numerical model simulation. Observations show that a separation between the wind shift and thermodynamic properties of the front was induced by the formation of a bore over south-central Kansas around 0200 UTC 29 November 2006. By the time the front–bore complex passed through Lamont, Oklahoma, approximately 4 h later, the bore had reached its maximum intensity and its associated wind shift preceded the trailing baroclinic zone by 20 min. Within several hours the bore decayed and a cold frontal passage, characterized by a wind shift coincident with thermodynamic properties was observed at Okmulgee, Oklahoma. Thus, a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relationship to the parent surface front occurred during a short period of time.

The details of this evolution are examined using output from a finescale numerical simulation, performed using the Weather Research and Forecasting (WRF) model. Analysis of the output reveals that as the bore advanced southeastward it moved into a region with a weaker surface stable layer. Consequently, the wave duct that had supported its maintenance steadily weakened resulting in dissipation of the bore. This circumstance led to a merger of the surface temperature and moisture boundaries with the orphaned wind shift, resulting in the cold frontal passage observed at Okmulgee.

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Brian J. Carroll, Belay B. Demoz, David D. Turner, and Ruben Delgado

Abstract

The 2015 Plains Elevated Convection at Night (PECAN) field campaign provided a wealth of intensive observations for improving understanding of interplay between the Great Plains low-level jet (LLJ), mesoscale convective systems (MCSs), and other phenomena in the nocturnal boundary layer. This case study utilizes PECAN ground-based Doppler and water vapor lidar and airborne water vapor lidar observations for a detailed examination of water vapor transport in the Great Plains. The chosen case, 11 July 2015, featured a strong LLJ that helped sustain an MCS overnight. The lidars resolved boundary layer moisture being transported northward, leading to a large increase in water vapor in the lowest several hundred meters above the surface in northern Kansas. A branch of nocturnal convection initiated coincident with the observed maximum water vapor flux. Radiosondes confirmed an increase in convective potential within the LLJ layer. Moist static energy (MSE) growth was generated by increasing moisture in spite of a temperature decrease in the LLJ layer. This unique dataset is also used to evaluate the Rapid Refresh (RAP) analysis model performance, comparing model output against the continuous lidar profiles of water vapor and wind. While the RAP analysis captured the large-scale trends, errors in water vapor mixing ratio were found ranging from 0 to 2 g kg−1 at the ground-based lidar sites. Comparison with the airborne lidar throughout the PECAN domain yielded an RMSE of 1.14 g kg−1 in the planetary boundary layer. These errors mostly manifested as contiguous dry or wet regions spanning spatial scales on the order of ten to hundreds of kilometers.

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Daniel C. Hartung, Jason A. Otkin, Ralph A. Petersen, David D. Turner, and Wayne F. Feltz

Abstract

In this study, atmospheric analyses obtained through assimilation of temperature, water vapor, and wind profiles from a potential network of ground-based remote sensing boundary layer profiling instruments were used to generate short-range ensemble forecasts for each assimilation experiment performed in Part I. Remote sensing systems evaluated during this study include the Doppler wind lidar (DWL), Raman lidar (RAM), microwave radiometer (MWR), and the Atmospheric Emitted Radiance Interferometer (AERI). Overall, the results show that the most accurate forecasts were achieved when mass (temperature and humidity profiles from the RAM, MWR, and/or AERI) and momentum (wind profiles from the DWL) observations were assimilated simultaneously, which is consistent with the main conclusion from Part I. For instance, the improved wind and moisture analyses obtained through assimilation of these observations contributed to more accurate forecasts of moisture flux convergence and the intensity and location of accumulated precipitation (ACPC) due to improved dynamical forcing and mesoscale boundary layer thermodynamic structure. An object-based verification tool was also used to assess the skill of the ACPC forecasts. Overall, total interest values for ACPC matched objects, along with traditional forecast skill statistics like the equitable threat score and critical success index, were most improved in the multisensor assimilation cases.

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Wayne M. Angevine, Joseph Olson, Jake J. Gristey, Ian Glenn, Graham Feingold, and David D. Turner

Abstract

Proper behavior of physics parameterizations in numerical models at grid sizes of order 1 km is a topic of current research. Modifications to parameterization schemes to accommodate varying grid sizes are termed “scale aware.” The general problem of grids on which a physical process is partially resolved is called the “gray zone” or “terra incognita.” Here we examine features of the Mellor–Yamada–Nakanishi–Niino (MYNN) boundary layer scheme with eddy diffusivity and mass flux (EDMF) that were intended to provide scale awareness, as implemented in WRF, version 4.1. Scale awareness is provided by reducing the intensity of nonlocal components of the vertical mixing in the scheme as the grid size decreases. However, we find that the scale-aware features cause poorer performance in our tests on a 600-m grid. The resolved circulations on the 600-m grid have different temporal and spatial scales than are found in large-eddy simulations of the same cases, for reasons that are well understood theoretically and are described in the literature. The circulations [model convectively induced secondary circulations (M-CISCs)] depend on the grid size and on details of the model numerics. We conclude that scale awareness should be based on effective resolution, and not on grid size, and that the gray-zone problem for boundary layer turbulence and shallow cumulus cannot be solved simply by reducing the intensity of the parameterization. Parameterizations with different characteristics may lead to different conclusions.

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Julio C. Marín, Diana Pozo, Eli Mlawer, David D. Turner, and Michel Curé

Abstract

The Radiative Heating in Underexplored Bands Campaign (RHUBC-II) project was held from August to October 2009 in the Atacama Desert in Chile at 5320-m altitude. Observations from this experiment and a high-resolution numerical simulation with the Weather Research and Forecasting Model (WRF) were used to understand the structure and evolution of the atmosphere over a region with complex terrain and extremely dry environmental conditions. The mechanisms driving the local circulations during synoptically unperturbed conditions at the field site were studied. The study suggests that the field site is mainly affected by a mountain-scale and a plateau-scale thermally driven circulation. The latter seems to dominate. The advection of warm air by downslope flows from higher heights during nighttime may be the mechanism that counteracts the longwave radiative cooling at the surface, causing a small decrease of near-surface temperature during the night. WRF represents the near-surface and upper atmosphere reasonably well above the RHUBC-II site. Important orographic features are misrepresented in the model terrain, which may cause the observed differences in near-surface winds. The zonal pressure gradient between both sides of the mountain and the static stability of the air mass on the windward side of the terrain control the local circulations over the field site. Consequently, a misrepresentation of these mechanisms in the model may cause differences between the simulated winds and observations.

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Paul M. Markowski, Nathan T. Lis, David D. Turner, Temple R. Lee, and Michael S. Buban

Abstract

Observations of near-surface vertical wind profiles and vertical momentum fluxes obtained from a Doppler lidar and instrumented towers deployed during VORTEX-SE in the spring of 2017 are analyzed. In particular, departures from the predictions of Monin–Obukhov similarity theory (MOST) are documented on thunderstorm days, both in the warm air masses ahead of storms and within the cool outflow of storms, where MOST assumptions (e.g., horizontal homogeneity and a steady state) are least credible. In these regions, it is found that the nondimensional vertical wind shear near the surface commonly exceeds predictions by MOST. The departures from MOST have implications for the specification of the lower boundary condition in numerical simulations of convective storms. Documenting departures from MOST is a necessary first-step toward improving the lower boundary condition and parameterization of near-surface turbulence (“wall models”) in storm simulations.

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